1. Field of the Invention
The present invention relates to a heat resistant composition, and more particularly to catalysts and catalyst carriers for use in catalytic reactions such as catalytic combustion, etc., and a method of producing the same.
2. Related Art Statement
Catalysts are commercially used at operating temperatures of, for example, 200.degree.-600.degree. C. in usual petrochemical processes, and at about 750.degree. C. in steam reforming process which uses the highest temperature for the catalysts. Recently, catalytic converters have been used for purifying exhaust gases from automobiles, and the temperature of the converters reaches to about 850.degree. C. at the maximum. Further, catalysts have been interested for use at high temperatures in methanation reaction and catalytic combustion reaction for gas turbines, boilers and jet engines, and are expected in some cases to keep their activity even at high temperatures exceeding 1000.degree. C. Under these circumstances, development of catalysts or catalyst carriers of excellent heat resistant property, i.e., property to keep its high specific surface area even at high temperatures as high as possible, has been earnestly desired.
A catalytic combustion process in which a catalyst promotes reaction between fuel and oxygen has the following many advantages as compared with conventional non-catalytic combustion processes, so that it has attracted attentions.
(1) Complete combustion can be attained even at low temperatures. PA0 (2) Complete and stable combustion can be attained over a wide range of fuel/air ratio. PA0 (3) Generation of thermal NOx is greatly diminished.
In order to establish a technique of catalytic combustion process, it is essential to develop a catalyst which has activity at low temperatures and can keep its activity as high as possible even at high temperatures. For this purpose, many efforts have been made in many places, at present. Because adoption of a heat resistant catalyst carrier is the most effective means for developing heat resistant catalysts, a heat resistant carrier has attracted researchers' especial interests nowadays.
Among the catalyst carriers which has been used commercially, alumina is known to be most heat resistant. .gamma.-Alumina is a carrier which has a specific surface area of about 200 m.sup.2 /g and keeps high specific surface area of more than about 50 m.sup.2 /g even when used at about 1000.degree. C. However, if used at 1200.degree. C., it is transited to .alpha.-alumina and rapid sintering occurs to decrease its specific surface area to about 5 m.sup.2 /g or less. If it is used at 1300.degree. C., the specific surface area of the used alumina becomes less than 1 m.sup.2 /g which has no significance as a catalyst carrier. Thus, in order to improve heat resistant property of alumina at high temperature, many attempts have been made to add a second component to alumina and to make complexes thereof.
It is known to improve catalyst by addition of an alkaline earth metal oxide to a nickel catalyst supported on alumina carrier. For example, Japanese patent application publication No. 44-17,737 describes "it is instructed in specification of British Pat. No. 969,637 that the proportion of the alkaline compound is preferably within the range of 0.75 to 8.6% calculated as metal on the combined weight of metallic nickel and alumina, and is larger than the rate of alumina in the catalyst", "We found that the especially effective catalyst might be obtained when proportion of the alkaline earth metal is higher than that cited in the above mentioned specification", "The alkaline earth metals which can be used are barium, strontium, calcium and magnesium but barium is the most preferable", and "Especially preferable rate of barium metal is 10 to 20 weight % to the aggregated weight of nickel, alumina and barium".
This especially preferable rate of 10-20 wt% of barium metal corresponds to Examples 3, 4 and 5 of the above Japanese publication, and the amounts of barium metal in Examples 3, 4 and 5 are respectively about 30 moles, 44.5 moles and 74 moles when calculated per 100 moles of alumina, so that they are very large. In addition, the purpose of adding the alkaline earth metal compound is not the improvement of its heat resistant property, but is clearly to exhibit an effect of a promoter to prevent formation of carbon on the catalyst surface. Further, use temperature of the catalyst is limited to not over than 600.degree. C., so that it is not directed to improvement of heat resistant property of catalyst at high temperatures of 1200.degree. C. or more as the present invention aims.
In general, as catalyst carriers for catalytic combustion process etc., alumina carriers or other carriers of comparatively large specific surface area have been used, and studied. However, at use conditions of high temperatures exceeding 1100.degree. C., especially exceeding 1200.degree. C., specific surface area of the carriers deteriorates rapidly, so that the carriers have a drawback of decreasing their activity, particularly when used again at low temperatures. Therefore, catalysts or catalyst carriers have been heretofore desired which have a property of retaining their specific surface area as high as possible and are not fragile even at high temperatures. Hereinafter, such property is called as "good heat resistant property".
There were proposed many methods of improving the heat resistant property of catalyst carrier made of alumina. For example, U.S. Pat. No. Re. 30,608, U.S. Pat. No. 4,013,587 and U.S. Pat. No. 4,013,590 proposed an addition of silica to alumina, and Japanese patent application laid-open No. 48-14,600 proposed an addition of rare earth oxide to alumina. However, neither of these methods could prevent the transition of .gamma.-alumina to .alpha.-alumina at temperatures exceeding 1200.degree. C., so that the decrease of the activity and fragilization or weakening of catalyst particles were unavoidable.